📌 Key Points
- Physical changes affect only the appearance (size, shape, colour, state) of a substance. No new substance is formed and they are generally reversible.
- Chemical changes alter the internal structure of a substance to form new substance(s) with different properties. They are always irreversible.
- A chemical reaction involves reactants (substances that change) and products (new substances formed). Represented as: Reactants → Products.
- Signs of a chemical reaction: change in colour, evolution of gas, change in temperature, formation of precipitate, and change in smell.
- Combination reaction: Two or more substances combine to form a single product. General form: A + B → AB.
- Decomposition reaction: A single compound breaks down into two or more simpler substances on supplying energy. General form: AB → A + B.
- Displacement reaction: A more reactive element displaces a less reactive element from its compound. General form: A + BC → AC + B.
- Oxidation: Addition of oxygen or removal of hydrogen from a substance.
- Reduction: Removal of oxygen or addition of hydrogen to a substance. Oxidation and reduction always occur simultaneously.
- Oxidising agent causes oxidation (and itself gets reduced). Reducing agent causes reduction (and itself gets oxidised).
- Exothermic reactions release heat (e.g., burning, respiration). Endothermic reactions absorb heat (e.g., decomposition of NaHCO3, photosynthesis).
- Rusting requires both oxygen and moisture. Equation: 4Fe + 3O2 + 6H2O → 4Fe(OH)3. It is an oxidation reaction.
- Rusting can be prevented by painting, oiling/greasing, galvanisation (zinc coating), and alloying (stainless steel).
- Burning of a candle is both physical change (wax melts) and chemical change (wax burns to form CO2 and H2O).
- Crystallisation is a physical change - it changes shape, size, and state but not chemical composition.
📘 Important Definitions
🔢 Formulas & Laws
Rusting of Iron
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Iron + Oxygen + Water → Iron hydroxide (rust). Requires both oxygen and moisture.
Burning of Carbon (Combination)
C + O2 → CO2
Carbon combines with oxygen. Also an oxidation and exothermic reaction.
Decomposition of Sodium Bicarbonate
2NaHCO3 → Na2CO3 + H2O + CO2
Sodium bicarbonate decomposes on heating. An endothermic reaction.
Decomposition of Calcium Carbonate
CaCO3 → CaO + CO2
Calcium carbonate decomposes on heating to form quicklime and carbon dioxide.
Displacement of Copper by Iron
Fe + CuSO4 → FeSO4 + Cu
Iron displaces copper from copper sulphate. Blue solution turns green.
Lime Water Test for CO2
Ca(OH)2 + CO2 → CaCO3 + H2O
CO2 turns lime water milky due to formation of calcium carbonate precipitate.
Reduction of Magnesium Oxide
MgO + C → Mg + CO
MgO is reduced (loses oxygen), carbon is oxidised (gains oxygen). Carbon is the reducing agent.
⚠️ Common Mistakes
✗ Wrong: Dissolving sugar in water is a chemical change.
✓ Correct: Dissolving sugar in water is a physical change because no new substance is formed. Sugar can be recovered by evaporating the water.
✗ Wrong: Oxidation only means addition of oxygen.
✓ Correct: Oxidation means addition of oxygen OR removal of hydrogen. Similarly, reduction means removal of oxygen OR addition of hydrogen.
✗ Wrong: All chemical reactions release heat.
✓ Correct: Only exothermic reactions release heat. Endothermic reactions absorb heat from the surroundings. Example: decomposition of NaHCO3 absorbs heat.
✗ Wrong: Iron rusts when exposed to dry air.
✓ Correct: Iron requires BOTH oxygen and moisture to rust. In completely dry air or in water without dissolved oxygen, iron does not rust.
✗ Wrong: Crystallisation is a chemical change because the substance changes form.
✓ Correct: Crystallisation is a physical change. It changes the shape, size, and state of a substance but NOT its chemical composition.
✗ Wrong: The oxidising agent gets oxidised in a reaction.
✓ Correct: The oxidising agent gets REDUCED (not oxidised). It causes oxidation of other substances but itself undergoes reduction. Similarly, the reducing agent gets oxidised.
📝 Exam Focus
These questions are frequently asked in CBSE exams:
Diagram to practice: Practice drawing: (1) Table comparing physical and chemical changes with examples, (2) Table comparing exothermic and endothermic reactions, (3) Diagram showing iron nail in copper sulphate solution (displacement reaction with colour change).
🎯 Last-Minute Recall
Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!